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1999 OBW 2.5l  A.T.

had a low idle (below 700 rpm) & sometimes wouldn't start right up after a short stop (but it always started 2nd time). So, I risked my knuckles & replaced the spark plugs & wires (double platinum NGK/mid priced house wires). ran smooth but noticed that at a light it would idle smoothly, then suddenly shudder, & then resume a good idle (still low, though)... it is consistent problem. The problem is much less noticable if I put the car in neutral.

I have replaced the PCV valve, the air filter, the fuel filter & I can find no vacuum leaks... no difference.

It also seems to have a lot less pick up than my current wife's '02 Impreza Outback Sport... (that goes like a bat outta hell)... but it never did. It has plenty of power for long trips & can cruise smoothly at 80 mph... although I *never* do that.

any help, suggestions, ideas... wisdom?

later,

Peter

Edited by later_Peter

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how many miles?

 

 

have you done a throttlebody cleaning? There's a spray Seafoam with a special nozzle that will clean the throttlebody ans help clean-out injectors/carbon in combustion chamber.

 

or, try a can of seafoam or techron in a tank or 2 of gas.

 

CEL on? what's the code?

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how many miles on the car? is it due for a timing belt? could have slipped timing by a tooth.

 

I know you said you could not find a vacuum leak - but you might try using a vacuum gauge to see what it looks like when it stumbles at idle.

 

I wouldn't discount the possibility of a bad knock sensor either since you have low power.

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knock sensor replaced about 3 years ago (threw a CEL)

192k on engine

Timing belt (et al) replaced around 140k

I just Seafoam DeepCreeped the throttle body... may have made a bit of difference. Still only stutters with foot on the brake while in drive... stop lights & such...

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Still only stutters with foot on the brake while in drive... stop lights & such...

wait - if you slip the car into neutral at a light, does the vibration stop? This is often a symptom of poorly rebuilt front half-axles. I haven't read any reports of it being 'cyclical' though i suppose it's possible.

 

Has the transmission fluid been changed on schedule? How does it look?

Edited by 1 Lucky Texan

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seriously, actually look at the engine while idling and a helper puts it in drive, then in neutral. The load on the engine will change a little with the torque converter but, again, there are plenty of complaints that the vibration is NOT from the engine missing - it's from bad axles.

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ok used the Seafoam spray "Deep Creep" on throttle body & "stumble" lessened. I decided to re-treat it so I actually removed the T.B. & cleaned it front & back with the spray & a toothbrush (hope my current wife doesn't notice)... ran really poorly... drove around again & seemed to be back to slight stumble. Been reading some stuff, since the idle is around 5/6k... figuring if I could raise the idle, maybe... Well, you can't. maybe the Idle Air Control Valve... seems like an easy replacement (except in the wallet area). Would a wrecking yard IACV be worth while? I could remove it, clean it, & replace it... but am I trading a stomach ache for a headache? OR live with the inconsistent idle?

191k

2.5L

'99 OBW

 

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Man I dunno, try scanning for 'pending' codes. I know you said vacuum was good, but a vacuum gauge can sometimes help pin down a problem. maybe do a compression check.

 

many people here have trouble with spark plug wires that are not OEM/NGK.

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you know, since it started right after I changed the plugs & wires, the wires may just be the issue... it's just such a PIA to change them out... but that'll be next, I think... thanks for the clue, Mr. Texan, sir!

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Dirty IAC is always a candidate for a low idle, and may cause idle speed to "hunt" occasionally, drop to about 500 rpm then rebound to ~1000 and back and forth. My car does this, but only when its hot outside and the engine is warm, usually only after restarting, or when the AC is on.

IAC is easy to clean. Remove the 1" hose and spray the valve while flicking it back and forth with a finger or small screwdriver.

 

Does it matter if the engine is cold or warm? A misfire that is not present when cold could be due to a bad front O2 sensor.

 

Not sure why I missed this thread before.

  • Like 1

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thanks Fairtax... When the engine is cold, the idle speed is much higher & there's no "stutter"... I will try to clean the IACV tomorrow (daylight)... I didn't think about the O2 sensor... maybe I'll replace that thing & see. I did replace one of the replacement spark plug wires with an old one... idle seemed to change *slightly*... more of an OCD type of annoyance, the car runs well & idles in neutral/park... it's just this one "stumble" issue.

later,

Peter
 

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How often does this stumble occur? Once every few seconds? Is it just a quick putt every once in a while like a misfire?

Or is it a chug chug chug kinda stumble? Lasts about 1-2 seconds and the engine speed drops to ~500 rpm (or less?) then bounces back to 750?

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hey thanks once again:
idle is around 600/650 while in drive with a foot on the brake

stumble is an occasional sagging in the power (idle hardly has a chance to drop) maybe once or twice during a red light. Sporadic, can't attach it to a cylinder fire on a regular basis...

hope this helps you help me... (I do appreciate it)

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Just so that we can be relatively assured of the engine speed reading I'm gonna bring this up because it has caused confusion in the past. The tachometer in Subarus can be decieving since it only has two tick marks between 0 and 1000 rpm. Often regarded as a 1/3s scale rather than 1/4s scale like the rest of the guage.

The first mark, the larger of the two, is 500 rpm, second mark is 750. Sometimes this second mark gets read as being around 600-650 (roughly 2/3rds of 1000).

No real point in the tachometer reading below 500 since the engine will never be operating in the 0-500 rpm range. If engine speed drops much below 500 the engine is likely to stall even if under no load.

 

Normal engine idle speed for an AT car is 775-800 rpm in park/neutral, 725-750 in gear. Which puts the needle right at or a hair below that second tick mark.

 

If you already knew this, good, and we can chase after the cause of the low idle speed at 600 rpm.

If this is news, also good because then you don't have to fix a low idle problem! Just need to figure out why its st-st-stuttering.

 

Is the AC on when this stumbling occurs? Does turning the AC off have any effect?

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well, well & well... I guess the idle may be correct!!! (I'll get the "ol' tachometer" out & verify)...

A/C on & the idle goes up & the "stutter" smooths out

 

once again, Mr. Fairtax, I thank you & I am impressed!

(btw, the Washington DC license plate says "taxation without representation" & thought of you... my friend just returned from there)

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Yep, DC tags are a common sight here, even being 3 hours away.

 

If you have an old dwell tach it probably won't read correctly on the wasted spark system used in the Subaru. This system is one of the reasons (actually the main reason) these cars are so picky about spark plugs and wires.

 

I've seen platinum type spark plugs cause problems on these engines, but it's usually a constant misfire, or a misfire under load. It is conceivable that they could cause a minor issue such as what you're experiencing. I've never gone wrong using standard copper in these cars. I'd suggest swapping the platinums for a set of NGK V power coppers (the $2.99 a piece ones).

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so apparently, here's the upshot:

cleaned throttle body

cleaned idle control valve

cleaned egr valve

through a P0400 code again

so replaced (junk yard) egr valve

& then noticed:
SPARK PLUG WIRE WAS LOOSE!!!

problem resolved...

(feeling pretty stupid now)

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